class Solution {
public:
    bool isUnique(string astr)
    {
        //λͼ
        int bitpho = 0;

        for (auto ch : astr)
        {
            int x = ch - 'a';
            if ((1 & (bitpho >> x)) == 1) return false;
            else bitpho |= 1 << x;
        }

        return true;
    }
};